The invention relates to a method for controlling the lean operation of an internal combustion engine, especially an internal combustion engine of a motor vehicle, provided with a nitrogen oxide storage catalyst.
In current automotive engineering, spark ignition engines as internal combustion engines with direct gasoline injection instead of conventional intake pipe injection are preferred, since these internal combustion engines compared to conventional spark ignition engines have distinctly more dynamics, are superior with respect to torque and power, and at the same time facilitate a reduction of fuel consumption by up to 15%. This is made possible mainly by so-called stratified charging in the partial load range in which only in the area of the spark plug an ignitable mixture is necessary, while the remaining combustion space is filled with air. As a result, the engine can be operated unchoked; this leads to reduced charge changes. In addition, the direct gasoline injector benefits from reduced heat losses, since the air layers around the mixture cloud are insulated toward the cylinder and the cylinder head. Since conventional internal combustion engines, which work according to the intake manifold principle, can no longer be ignited at such a high air excess as is present in direct gasoline injection, in this stratified charging mode the fuel mixture is concentrated around the spark plug which is positioned centrally in the combustion space, while in the edge areas of the combustion space there is pure air. In order to be able to concentrate the fuel mixture around the central spark plug which is positioned in the combustion space, a concerted air flow in the combustion space is necessary, a so-called tumble flow. In the process, an intense, roller-shaped flow is formed in the combustion space and the fuel is injected only in the last third of the upward motion of the piston. By the combination of a concerted air flow and special geometry of the piston, which for example has for example a pronounced fuel and flow depression, the especially finely dispersed fuel is concentrated in a so-called “mixture ball” ideally around the spark plug and ignites reliably. The engine control provides for the respectively optimized adaptation of the injection parameters (point of injection time, fuel pressure).
These internal combustion engines can therefore be operated in lean operation for a correspondingly long time; this benefits fuel consumption overall, as has been described in the foregoing. This lean operation however entails the disadvantage that the nitrogen oxides (NOx) in lean exhaust cannot be reduced by a 3-way catalyst. In order to keep the nitrogen oxide emissions within the scope of prescribed limits, for example of the Euro-IV limit value, nitrogen oxide storage catalysts are generally used in conjunction with these internal combustion engines. These nitrogen oxide storage catalysts are operated such that the nitrogen oxides produced by the internal combustion engine in a first phase of operation as the lean operating phase are stored in the nitrogen oxide storage catalyst. This first operating phase or lean operating phase of the nitrogen oxide storage catalyst is also called the storage phase. As the length of the storage phase increases, the efficiency of the nitrogen oxide storage catalyst decreases; this leads to an increase of nitrogen oxide emissions downstream of the nitrogen oxide storage catalyst. The reduction in efficiency is caused by the increase in the nitrogen oxide fill level of the nitrogen oxide storage catalyst. The rise in nitrogen oxide emissions downstream of the nitrogen oxide storage catalyst can be monitored and after a definable threshold value is exceeded, a second operating phase of the nitrogen oxide storage catalyst, a so-called discharge phase, can be initiated. During this second operating phase, in the exhaust of the internal combustion engine a reducing agent is added which reduces the stored nitrogen oxides to nitrogen and oxygen. The reducing agent is generally a hydrocarbon (HC) and/or carbon monoxide (CO) which can be produced in the exhaust gas simply by a rich setting of the fuel/air mixture. Towards the end of the discharge phase, most of the stored nitrogen oxide is reduced and less and less of the reducing agent meets the nitrogen oxide which it can reduce to oxygen and nitrogen. Towards the end of the discharge phase the proportion of the reducing agent in the exhaust gas downstream of the nitrogen oxide storage catalyst therefore rises. By corresponding analysis of the exhaust downstream of the nitrogen oxide storage catalyst, for example by means of an oxygen sensor, the end of the discharge phase can then be initiated and it becomes possible to switch again to the lean operation phase. In the disclosed nitrogen oxide storage catalysts this switching is carried out at time intervals of for example 30 to 60 seconds, the regeneration, i.e., the discharge phase, lasting approximately 2 to 4 seconds.
The problem however is that in nitrogen oxide storage catalysts with increasing service life the storage capacity for nitrogen oxides decreases. This is due to the fact that mainly the sulfur contained in fuels leads to poisoning of the storage catalyst, i.e., to permanent deposition of sulfur in the storage catalyst which reduces the storage capacity for nitrogen oxides. The nitrogen oxides are stored in nitrogen oxide storage catalysts in the form of nitrates, while sulfur is stored in the form of sulfates. Since sulfates are chemically more stable than nitrates, the sulfate cannot decompose in nitrogen oxide regeneration. Only at catalyst temperatures above 650° C. under reducing conditions can sulfur be discharged. Such high catalyst temperatures are generally not reached however, especially in city traffic.
The generic WO 02/14658 A1 discloses a process for controlling lean operation of an internal combustion engine having a nitrogen oxide storage catalyst, in which the nitrogen oxides produced by the internal combustion engine in a first operating phase (lean operation) as the storage phase are stored for a specific storage time in the nitrogen oxide storage catalyst, and in which after the storage time expires, by a control device as the engine control at a specific switching instant for a specific discharge time switching to a second operating phase (rich operation) takes place as the discharge phase in which the nitrogen oxides which have been stored during the storage time are discharged from the nitrogen oxide storage catalyst. Furthermore, the nitrogen oxide mass flow upstream of the nitrogen oxide storage catalyst and/or the nitrogen oxide mass flow downstream of the nitrogen oxide storage catalyst are each integrated over the same time interval.
Specifically, here these integral values are put into a relative relationship to one another. Thus, in this process a quality factor will be determined which renders possible a conclusion about the storage capacity of the nitrogen oxide storage catalyst with respect to ageing of the catalyst by sulfur poisoning and thermal damage or the age-induced decrease of the storage capacity. In particular, in this process the degree of poisoning of the catalyst with sulfur will be determined and thus the sulfur content in the control device of the internal combustion engine will be corrected in order to optimize sulfur regeneration. By integration over the time interval, the effects of fluctuations and disruptions on the determined nitrogen oxide mass flow values will be reduced since, viewed over a specific time interval, a type of average value of the quality factor is obtained which will be more conclusive than the individual instantaneous values obtained at specific times. But in practical operation for nitrogen oxide storage catalysts in general such complex operating conditions prevail that the quality factor, in spite of the reference to a specific time interval, under certain circumstances does not adequately reproduce the actual state of the storage capacity of the nitrogen oxide storage catalyst. This can on the one hand have an adverse effect on fuel consumption, since for example a rich mixture is supplied too early. On the other hand, there is the danger that the savings potential by lean operation is so low that only a small advantage in fuel consumption can be gained. Since lean operation however leads to high nitrogen oxide emissions, in certain operating ranges the advantage in fuel consumption is not in a favorable ratio to the actual nitrogen oxide emissions. Discharge itself in this procedure will only take place when the modeled, stored nitrogen oxide mass has exceeded a specific boundary value.
Furthermore, in conjunction with operation of the nitrogen oxide storage catalyst, consideration of ageing, especially the ageing of sulfur poisoning, in the design of a nitrogen oxide storage catalyst is to be taken into consideration in order to ensure that the ageing of the catalyst over the intended service life of the catalyst leads to adherence to given exhaust boundary values with respect to nitrogen oxide emissions in an aged nitrogen oxide storage catalyst. In this respect, adapting the number of discharges to the amount of nitrogen oxide discharged per charging and discharging cycle is generally known such that at a storage capacity of the aged nitrogen oxide storage catalyst which has been reduced relative to a new nitrogen oxide storage catalyst the amount of nitrogen oxide released during the exhaust gas test time interval does not exceed the given exhaust boundary value. This amount of nitrogen oxide which is released per charging cycle for an aged storage catalyst is an absolute quantity and constitutes the absolute nitrogen oxide slip, i.e., that as soon as the storage catalyst is charged with this amount of nitrogen oxide, discharge takes place. This absolute nitrogen oxide slip as an established value applies both to the new and also the aged nitrogen oxide storage catalyst. Since a rich mixture of lambda quantity 1 is required per discharge, with an increasing number of discharges in the course of ageing of a storage catalyst the fuel consumption also rises compared to that of a new storage catalyst.